A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate, to create a device. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is exposed by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In most cases, the process of exposing the target portions is repeated a plurality of times, thereby generating a device comprising a plurality of layers. In order for proper operation of the device, an accurate positioning of the layers relative to each other is needed. As such, during the exposure process, the position of the substrate relative to the patterning device needs to be accurate. In order to determine this position, a lithographic apparatus in general comprises a position measurement system such as an interferometer based measurement system or an encoder based measurement system. Such systems can e.g. be used for determining a position of a support holding the patterning device relative to a position of a support holding the substrate. Such systems generally have a position sensor target and a position sensor. The position measurement system determines the position of a support by determining a displacement between the position sensor target and the position sensor. Based on the displacement, the position measurement system creates a position signal.
As will be understood by the skilled person, an accurate positioning of the support using the position measurement system, relies on the position signal provided by the position measurement system. However, the displacement between the position sensor target and the position sensor may not be an accurate measure of the position of the substrate relative to the patterning device. The accuracy may deteriorate due to deformations of the supports or the position measurement device. An undesired displacement of the position sensor and position sensor target, for example due to thermal drift, could also deteriorate the accuracy.
In order to accommodate for such a deformation or displacement, a periodic calibration of the position measurement system is often applied. In general, such a calibration may be time-consuming and may result in an important down-time of the apparatus, thus adversely affecting the productivity of the apparatus.